Slow boat contrary to expectation capsized (3)
I believe the answer is:
tub
'slow boat' is the definition.
(tub can mean a rudimentary boat)
'contrary to expectation capsized' is the wordplay.
'contrary to expectation' becomes 'but' ('but...' can mean 'contrary to expectation').
'capsized' says the letters should be written in reverse.
'but' backwards is 'TUB'.
(Other definitions for tub that I've seen before include "Wooden bath or slow boat" , "Container, usually for washing" , "Bath; ice cream container" , "Unseaworthy vessel" , "Maybe it sailed" .)